The quinone/hydroquinone redox couple is used in many different technologies and has been extensively studied. In U.S. Patent Publication No. 2015/0263371, which is incorporated by reference herein in its entirety, we disclosed using the quinone/hydroquinone redox couple as a charge transfer mediator to facilitate more efficient electrocatalytic oxygen reduction in electrochemical cells. However, in the context of emerging electrochemical cell technologies, such as organic mediator flow batteries and mediated fuel cells, the available quinones are inadequate.
In the context of such technologies, effective redox mediators must have a reduction potential close to the thermodynamic potential for reduction of oxygen to water, high solubility in water, and stability in aqueous solutions under the conditions used in such applications. Although the unsubstituted hydroquinone/1,4-benzoquinone redox couple has a sufficiently high reduction potential in the oxidized form, it has relatively low solubility in water and is unstable in acid solution.
Hydroquinone can be sulfonated to yield useful compounds, such as the commercially available potassium hydroquinone monosulfonate. More vigorous sulfonation conditions give rise to the 2,5- and 2,6-disulfonated isomers.1 These sulfonate salts have high water solubility compared to the parent hydroquinone, and the solubility of the acid is even higher. Aerobic or electrochemical oxidation of these compounds produces the corresponding para-quinone. Sulfonation of catechol gives the 3,5-disulfonate, which can be oxidized to an ortho-quinone. These quinones have been proposed as redox-active species in flow batteries.2 
We undertook experiments on these quinones to determine their suitability for use in a mediated fuel cell. To our dismay, the mono- and di-substituted quinones described above all proved to be unstable in aqueous acid, even at low temperature (see Scheme 1). The condensation of the mono-sulfonated quinone is presumed to be analogous to previously studied decomposition reactions of benzoquinone and toluquinone.3 The presence of two sulfonate groups prevents polymerization, but addition of water still takes place, even in 1 M H2SO4. Although the resulting dihydroxyquinone disulfonates are stable in solution, their reduction potentials are too low to be useful in a flow battery or fuel cell. The addition of water has been since confirmed for the disulfonated ortho-quinone in a recent paper by Yang, et al.4

Accordingly, there is a need in the art for highly substituted hydroquinones/quinones with substituents that result in a quinone reduction potential at least as high as the reduction potential of unsubstituted benzoquinone, while also having the greater stability exhibited by low-potential polysubstituted quinones. In addition, there is a need in the art for improved methods for synthesizing such highly substituted hydroquinones/quinones. Water soluble substituted hydroquinones/quinones having such properties could function as improved redox mediators in electrochemical cells, particularly to facilitate oxygen reduction in mediated fuel cells or in organic-mediator flow batteries.